The opportunistic infections (OIs) resulting from advanced HIV disease are principal causes of morbidity and mortality. Several species of parasites belonging to the phylum Microspora cause disease in immunocompromised hosts, including people with AIDS. Microsporidia have only recently been recognized as human pathogens and opportunistic infections, thus the field is a very new area of study. AIDS patients commonly develop chronic diarrhea, and other clinical manifestations that may include keratitis, nephritis, pneumonia, hepatitis, peritonitis, and sinusitis. Gastrointestinal infections comprise the vast majority of microspordiosis cases. These is no therapy for Enterocytozoon bieneusi, the most common of the microsporidial infections causing a debilitating diarrhea. This RFP represents a segment of the Opportunistic Infections Research Branch's drug discovery and development program, through which drug testing resources are made available to scientific investigators and to companies without comparable resources. The availability of these test systems will allow DAIDS to provide needed support for investigator- initiated drug discovery, to stimulate private sector sponsorship of new drugs, to perform comparison studies of drugs from different sponsors, and to provide information for selection of anti-microsporidial drug candidates for design of clinical studies. At the present time, NIAID does not have any resource for testing therapies against microsporidia. The new contract resulting from this RFP will develop and validate in vitro and in vivo models of microsporidial infection that are suitable for testing new therapies. The in vitro and in vivo models established and validated through this contract will serve to test candidate antibiotics and provide a standard system to assist pathogenesis, parasite biology, drug resistance and diagnostics research. The state-of-the-art of research on this group of pathogens is such that a few species can be cultured and limited animal models are under development for Encephalitozoon-type species. A major obstacle to progress in this are is that there is no culture system or animal model of the most common microsporidial pathogen, Enterocytozoon bieneusi, to support pathogenesis and drug discovery research. Therefore the emphasis of this contract is to fill the gaps in knowledge and methodology regarding research on E. Bieneusi. Due to the current limitations of working with this organism, the first year of the contract will be used to further develop proposed in vitro and in vivo models of E. bieneusi infection. Evaluation of experimental therapies is anticipated for years 2-5, along with continued efforts to improve the models.